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Aquarium guides/Hydra in the aquarium

Hydra in the aquarium: tiny tentacled polyps on the glass

Hydra are tiny freshwater relatives of jellyfish: a stalk from a few millimeters to about 0.4 inch (10 mm) long when stretched out, with a crown of tentacles, stuck to glass, plants or decor. They sting and eat tiny animals, including newborn fry and baby shrimp, but cannot hurt adult fish. They multiply on surplus food, especially baby brine shrimp and powdered foods; feed less and they shrink back.

Guppy fry
Shown: guppy fry. Photo: Nevit Dilmen, CC BY-SA 3.0, via Wikimedia Commons

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Why they appear

Hydra arrive on plants or with live food and stay small until there is plenty to eat. A sudden spread almost always follows heavy feeding of fry foods, baby brine shrimp or powdered food. Feeding fry.

How to get rid of them

  1. Cut the feeding back to what the fish and shrimp eat in a few minutes.
  2. Scrape them off the glass and siphon them out during a water change.
  3. Some fish eat them, such as gouramis and some livebearers; this suits a community tank, not a fry or shrimp tank.
  4. Treatments are sold for hydra and planaria in shrimp tanks; some are not safe with snails, so read the label for every animal in the tank, move snails you want to keep to another tank first, and change water afterwards.

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